Alignment of carbon nanotubes

ABSTRACT

Carbon nanotubes are aligned within a host phase of a material that has molecules that will align under a certain influence. When the host molecules become aligned, they cause the carbon nanotube fibers to also become aligned in the same direction. The film of aligned carbon nanotubes is then cured into a permanent phase, which can then be polished to produce a thin film of commonly aligned carbon nanotube fibers for use within a field emission device.

TECHNICAL FIELD

[0001] The present invention relates in general to display systems, andin particular, to field emission displays.

BACKGROUND INFORMATION

[0002] Carbon nanotubes have been demonstrated to achieve good electronfield emission. However, in the prior art, the carbon nanotubes aredeposited on the cathode in disorganized positions. FIG. 1 illustratessuch a cathode 100 with a substrate 101 and an electrode 102.Illustrated are carbon nanotubes 103 deposited on electrode 102 in suchdisorganized positions. As a result of the random organization of thecarbon nanotube fibers, the efficiency of the electron emission isimpacted to be less than possible.

[0003] Therefore, there is a need in the art for a method of aligningsuch carbon nanotubes to improve the efficiency of the electron emissiontherefrom.

SUMMARY OF THE INVENTION

[0004] The present invention addresses the foregoing need by providing amethod for aligning carbon nanotubes within a host phase. Once thecarbon nanotubes are aligned, the host phase is then subjected to abinding process to make the alignment of the carbon nanotubes permanent.Thereafter, the surfaces of the host phase can be polished resulting insubstantially vertically aligned carbon nanotubes within a thin film,which can then be used within a cathode structure to produce a fieldemission device, including a display.

[0005] The foregoing has outlined rather broadly the features andtechnical advantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] For a more complete understanding of the present invention, andthe advantages thereof, reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings, inwhich:

[0007]FIG. 1 illustrates a prior art cathode using unaligned carbonnanotubes;

[0008]FIG. 2 illustrates carbon nanotubes aligned within a host phase;

[0009]FIG. 3 illustrates binding of the host phase;

[0010]FIG. 4 illustrates a thin film including vertically aligned carbonnanotubes;

[0011]FIG. 5 illustrates a field emission device using the thin film ofFIG. 4;

[0012]FIG. 6 illustrates a data processing system configured inaccordance with the present invention;

[0013]FIG. 7 illustrates a flow diagram of a process for aligning carbonnanotubes in accordance with the present invention;

[0014]FIG. 8 illustrates an alternative embodiment for the presentinvention;

[0015]FIG. 9 illustrates an etching step within an alternativeembodiment of the present invention;

[0016]FIG. 10 illustrates another etching step within an alternativeembodiment of the present invention;

[0017]FIG. 11 illustrates application of a metal layer on the hostphase; and

[0018]FIG. 12 illustrates an alternative embodiment of the presentinvention.

DETAILED DESCRIPTION

[0019] In the following description, numerous specific details are setforth such as specific host phases or display structures, etc. toprovide a thorough understanding of the present invention. However, itwill be obvious to those skilled in the art that the present inventionmay be practiced without such specific details. In other instances,well-known circuits have been shown in block diagram form in order notto obscure the present invention in unnecessary detail.

[0020] Refer now to the drawings wherein depicted elements are notnecessarily shown to scale and wherein like or similar elements aredesignated by the same reference numeral through the several views.

[0021] The present invention exploits the fact that carbon nanotubes aresimilar to elongated particles (molecules), which can be placed with ahost phase of ordered elongated particles. Such ordered elongatedparticles could be liquid crystals, ordered metal fibers in a liquidunder a magnetic or electric field, geometrically anizotropic particles,anizotropic crystals (elongated) possessing a strong dipole moment, etc.By selecting the size of the nanotubes with respect to the host phase,the present invention aligns the carbon nanotube fibers by aligning theparticles of the host phase.

[0022] Referring to FIGS. 2-4 and 7, as an example, the host phase 200could be a liquid crystal having liquid crystal molecules 205. Theliquid crystal can also include an ultraviolet (UV) curable binder thathardens the liquid crystal when exposed to UV light, as is furtherdiscussed below. The host phase may alternatively be a solution ofelongated crystals in an isotropic liquid medium (oil). Anotheralternative host phase would be a long chain of polymer moleculesaligned with each other through a mechanical means, such as rubbing.Such “rubbing” is a commonly used process within the liquid crystal art.Such a rubbing process is further discussed below. The carbon nanotubes204 are disposed within the host phase (step 701) and initially willlikely be unaligned with each other (not shown) similar to as that shownin FIG. 1. This is done within a container (not shown) betweenelectrodes 202 and 203. Electrode 203 is grounded while electrode 202 iscoupled to a power source 201. Assume for this example that the liquidcrystal molecules are long and heavy (≧500 angstroms). If the nanotubes204 are approximately 50 micrometers in length, a field of 50-60 voltswill align the host molecules 205 and eventually the nanotubes 204 (step702).

[0023] As an alternative, a substrate may be deposited at the bottom ofthe host phase 200 and above the electrode 203 so that the host phasewith the nanotubes is already deposited on a substrate instead ofperforming the mounting step 705 described below.

[0024] Another means for aligning the host phase is to place the hostphase in physical contact with an alignment layer, such as illustratedin FIG. 8. On a substrate 801, the alignment layer 802, which canconsist of long chain polymers in a semi-solid form are deposited, andthen rubbed or combed in one direction to align the polymers in aspecified direction. Physical contact of the host phase 803 with thealignment layer 802 aligns the molecules in the host phase in thespecified direction, this direction being dependent on many parameters.Alignment of the host phase in the specified direction induces alignmentof the nanotubes disposed within the host phase.

[0025] As noted previously, the host phase can contain an ultraviolet(UV) curable binder 302 (or other curable monomers, for example by heat,etc.). By shining an ultraviolet light, for example, on the organizedaligned phase 301, the process produces a solid film of aligned carbonnanotubes 204. This process is referred to as binding the alignment(step 703).

[0026] Thereafter, the solid film can be sliced, for example alongdashed lines A and B, and/or one or more of the surfaces polished (step704) to obtain a thin film 400 of organized carbon nanotubes to be usedas a cold electron source for field emission applications. Once anelectric field is produced, the carbon nanotubes 204 will emit fromtheir ends 401.

[0027] Referring to FIG. 9, step 704 may also alternatively include anetching phase, whereby a portion of the host phase 901 is etched backwithout etching the nanotubes. This is possible since the nanotubes aremade of a carbon or graphic material that is more resistant to etching.As a result, this process will expose portions of the nanotubes 902. Itshould be noted that the etching step can be performed in combinationwith or alternatively to the polishing process.

[0028] An alternative etching process is illustrated in FIG. 10, wherebya more directional etching process is performed, usually through the useof a mask (not shown), to selectively etch wells 1003 within the hostphase 1001 around selected carbon nanotubes 1002. Again, the result isthat portions of the nanotubes 1002 are exposed.

[0029] Another alternative embodiment of the present invention isillustrated in FIG. 12 where the nanotubes 1202 are contacted by aconductive layer 1205 on the bottom side. A conductive layer 1204 isdeposited on the top side. Wells 1203 are then etched down into the topside conductive layer 1204 and the host phase 1201 such that the topconductive layer 1204 is electrically isolated from the nanotubes 1202.Thus, the top conducting layer 1204 can be used as a gate control.

[0030] The exposing of the carbon nanotubes above the host phase canresult in a better emission of electrons from the carbon nanotubes.

[0031] As an alternative to providing a conductive layer on the bottomof the host phase, a conductive layer 1103 can be deposited on top ofthe host phase 1101 after an etching process to expose portions of thenanotubes 1102. Naturally, the conductive layer is used to produce theelectric field for emission of electrons from the carbon nanotubes 1102.

[0032] Alternatively, the host phase in each of the above embodimentscan be doped to make the host phase conducting or semiconducting, thuseliminating the need for a conductive layer.

[0033] This is further shown by the field emission device 500 is FIG. 5.An anode 501 is made of a substrate 502, an electrode 503 and a phosphor504. The cathode 505 includes a substrate 506, an electrode 507 and thethin film 400 discussed above. Upon the application of electric field,the carbon nanotubes will emit electrons. Any number of gate electrodesor extraction grids 508, 509 may optionally be implemented.

[0034] Such a field emission device 500 can be used in manyapplications, such as to produce single cathode pixel elements, toproduce large billboard-like displays, or even smaller displays such asfor computers. The cathodes may be aligned in strips to produce amatrix-addressable display.

[0035]FIG. 6 illustrates a data processing system 613 configured to usea display device made from the field emission devices described in FIG.5, which illustrates a typical hardware configuration of workstation 613in accordance with the subject invention having central processing unit(CPU) 610, such as a conventional microprocessor, and a number of otherunits interconnected via system bus 612. Workstation 613 includes randomaccess memory (RAM) 614, read only memory (ROM) 616, and input/output(I/O) adapter 618 for connecting peripheral devices such as disk units620 and tape drives 640 to bus 612, user interface adapter 622 forconnecting keyboard 624, mouse 626, and/or other user interface devicessuch as a touch screen device (not shown) to bus 612, communicationadapter 634 for connecting workstation 613 to a data processing network,and display adapter 636 for connecting bus 612 to display device 638.CPU 610 may include other circuitry not shown herein, which will includecircuitry commonly found within a microprocessor, e.g., execution unit,bus interface unit, arithmetic logic unit, etc. CPU 610 may also resideon a single integrated circuit.

[0036] Although the present invention and its advantages have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.

What is claimed is:
 1. A method for aligning geometric anisotropicparticles, comprising the steps of: disposing the geometric anisotropicparticles in a host phase; and causing the host phase to inducealignment of the geometric anisotropic particles.
 2. The method asrecited in claim 1, further comprising the step of: binding the alignedhost phase.
 3. The method as recited in claim 1, wherein the causingstep further induces molecules in the host phase to align whichphysically causes the geometric anisotropic particles to align with thehost phase molecules.
 4. The method as recited in claim 1, wherein thehost phase is aligned by an external force.
 5. The method as recited inclaim 4, wherein the external force is a mechanical force.
 6. The methodas recited in claim 4, wherein the external force is a magnetic force.7. The method as recited in claim 4, wherein the external force is anelectrical force.
 8. The method as recited in claim 4, wherein theexternal force is an optical force.
 9. The method as recited in claim 1,wherein the geometric anisotropic particles are elongated and alignedparallel to each other such that their long axes are parallel to eachother.
 10. The method as recited in claim 1, wherein the geometricanisotropic particles are carbon nanotubes.
 11. The method as recited inclaim 2, further comprising the step of polishing the binded and alignedhost phase.
 12. The method as recited in claim 2, further comprising thestep of etching the host phase to expose portions of the alignedgeometric anisotropic particles.
 13. The method as recited in claim 2,further comprising the step of depositing a conductive layer on the hostphase and around the exposed portions of the geometric anisotropicparticles.
 14. The method as recited in claim 1, wherein the host phaseis aligned by mechanical contact with an alignment layer.
 15. An articleof manufacture comprising: a host phase; and geometric anisotropicparticles disposed in the host phase, wherein the geometric anisotropicparticles are similarly aligned.
 16. The article of manufacture asrecited in claim 15, wherein the geometric anisotropic particles arecarbon nanotubes.
 17. The article of manufacture as recited in claim 16,wherein the carbon nanotubes are parallel to each other such that theirlong axes are parallel.
 18. A method of manufacturing a field emissiondevice, comprising the steps of: disposing carbon nanotubes into a hostphase; and causing the host phase to align resulting in alignment of thecarbon nanotubes.
 19. The method as recited in claim 18, furthercomprising the step of: binding the aligned host phase.
 20. The methodas recited in claim 19, wherein the causing step further causesmolecules in the host phase to align which physically causes the carbonnanotubes to align with the host phase molecules.
 21. The method asrecited in claim 19, further comprising the step of: slicing the hostphase substantially perpendicular to the alignment of the carbonnanotubes.
 22. The method as recited in claim 21, further comprising thestep of: polishing a surface of the host phase so that some of thecarbon nanotubes have their ends exposed.
 23. The method as recited inclaim 19, further comprising the step of: polishing a surface of thehost phase.
 24. The method as recited in claim 19, further comprisingthe step of: mounting the host phase with the aligned carbon nanotubesonto a substrate.
 25. The method as recited in claim 24, furthercomprising the steps of: disposing an anode opposite of the mounted hostphase; and operating an electric field between the anode and the mountedhost phase.
 26. The method as recited in claim 19, further comprisingthe step of etching the host phase to expose portions of the carbonnanotubes above the surface of the etched host phase.
 27. The method asrecited in claim 18, wherein the causing step further comprises the stepof depositing an alignment layer on a substrate and then disposing thehost phase onto the alignment layer in mechanical contact resulting inalignment of the carbon nanotubes disposed within the host phase. 28.The method as recited in claim 18, wherein the host phase is aligned byan external force.
 29. The method as recited in claim 27, wherein thealignment layer comprises long chain polymer molecules aligned with eachother.
 30. A field emission device comprising: a substrate; and a fieldemitter, comprising carbon nanotubes aligned in a host phase.
 31. Thedevice as recited in claim 30, further comprising: an electrode disposedbetween the field emitter and the substrate; an anode disposed adistance away from the field emitter, wherein the anode comprises aphosphor; and a power source operable for creating a electric field tocause electrons to emit from the carbon nanotubes towards the phosphor.32. The device as recited in claim 30, wherein portions of some of thecarbon nanotubes are exposed above the surface of the host phase. 33.The device as recited in claim 32, further comprising a conductive layerdisposed underneath the host phase and contacting some of the carbonnanotubes.
 34. The device as recited in claim 33, further comprising aconductive layer deposited on top of the host phase but not contactingthe exposed portions of the carbon nanotubes.
 35. A display comprising:an anode comprising a first substrate and a phosphor; a cathodecomprising a second substrate and field emitter, wherein the fieldemitter comprises a host phase with carbon nanotubes similarly alignedtherein; and circuitry for creating an electric field to cause electronsto emit from the carbon nanotubes towards the phosphor.
 36. A dataprocessing system comprising: a processor; a memory device; a storagedevice; and a display, wherein the processor is coupled to the memorydevice, the storage device, and display via a bus system, wherein thedisplay further comprises: an anode comprising a first substrate and aphosphor; a cathode comprising a second substrate and field emitter,wherein the field emitter comprises a host phase with carbon nanotubessimilarly aligned therein; and circuitry for creating an electric fieldto cause electrons to emit from the carbon nanotubes towards thephosphor.